In recent years considerable interest has been shown in the development of solid state lasers featuring high average optical output power. Laser systems such as Nd:YAG and Nd:Glass have received particular attention, as a candidate for a moderately short wavelength energy source, to be utilized for more efficient laser materials processing. However scaling to higher average power has proven to be very difficult, especially if high efficiency and good beam quality are to be maintained.
Two particularly difficult power limiting problems in these solid-state systems have been non-uniform excitation and non-optimum waste heat removal from the active media. Such non-uniformities in excitation and cooling induce refractive index inhomogeneity within the gain media, thereby leading to serious degradation in beam quality and possibly even to fracture of the solid state gain material. In an effort to ameliorate such difficulties, an number of novel geometries for pumping and cooling solid state laser gain material have been proposed and implemented. Of particular note have been the "axial-gradient", "segmented", "slab" and "zig-zag" configurations developed in the mid 70s. See: E. Matovich, "The Axial Gradient Laser", Proc., Fourth DOD Conf. Laser Tech., San Diego, Jan. 1970, Vol. 1, pp. 311-362; W. F. Hagen, C. G. Young, D. W. Cuff, and J. E. Keefe, "Segmented Nd Glass Laser", Proc., Fourth DOD Conf. Laser Tech., San Diego, Jan. 1970, pp. 363-377; J. M. Eggleston, T. J. Kane, J. Unternahrer, and R. L. Byer, "Slab-Geometry Nd:Glass Laser Performance Studies", Optics Letts. Vol. 7, pp. 405-407, 1982; G. J. Hulme, and W. B. Jones, "Total Internal Reflection Face Pumped Laser: Concepts and Design Considerations", SPIE, Vol. 69, pp. 38-44, 1975.
Similar configurations, and more recent inside-pumped approaches have raised the average output power extractable from Nd:YAG devices well into the 1 kilowatt regime. See: J. L. Emmett, W. F. Krupke and W. R. Sooy, "Potential of High Average-Power Solid State Lasers", Lawrence Livermore National Laboratory report # UCRL-53571 and UC-21,22; Walter Koechner, "Solid-State Laser Engineering", Springer-Verlag Series in Optical Sciences, Chap. 7, Spring-Verlag; U. Wittrock and H. Weber, "Inside-pumped Nd:Yag Tube Laser", Optics Letts. Vol. 16, pp. 1092-1094, Jul., 1991.
The attainment of even these modest power levels has however come at considerable cost, and usually also at the expense of mode quality. These aspects follow directly from the fact that high average power has hitherto required the growing of very large, single crystal rods, slabs, or cylinders, for the gain medium. In this context, experience has shown that the growing of large crystal units is not only very expensive, but that the optical quality in such a single crystal extended volume gain medium is also compromised. As such, near diffraction limited performance, under high average power operating conditions, is not achieved.
Indeed most high average power Nd:YAG lasers operate at more than 50 times their diffraction limit. This aspect seriously degrades the focusability and brightness of the beam; thereby greatly reducing the effectiveness of the laser for high speed, deep penetration materials processing.
There is disclosed here an alternative method for the attainment of high average optical power output from a solid state laser system. The technique, which utilizes a multi-channel approach for the establishment of the gain medium, appears equally applicable to a wide variety of optically-pumped gain material, either single crystal or glass. The approach is an extension of the multiple gain channel with common unstable optical extraction configuration developed previously by the same inventor for gas lasers: H. J. J. Seguin, "Laser System With Multiple Radial Discharge Channels", U.S. Pat. No. 5,029,173, Jul., 1991.
Thus in one embodiment, there is provided a laser system in which plural pairs of solid state gain channels are arranged about and extend radially from a first common central axis within an enclosure. The active media within the gain channels are cooled by conduction cooling, for example by the circulation of fluids within the enclosure. Means adjacent the gain channels provide uniform laser excitation energy to the gain media, such laser excitation energy being in the form of optical radiation derived from a plurality of strategically positioned flashlamps or arrays of light-emitting diodes.
Optical extraction means having a second axis coinciding with the gain media common central axis are disposed about the pairs of gain channels and generate a common resonator mode for all the gain channels and extract laser energy from all of the gain channels simultaneously.
The means for extracting the laser energy may include an optical resonator having a common unstable cavity mode and a segmented annular output, and may include an output axicon mounted to receive the segmented annular output, (one beamlet from each gain channel), and compact it into a continuous annular output beam.
Means for injecting an externally generated reference oscillator signal provides simultaneous phase-locking of the plurality of gain channels, thereby yielding an a externally-injection-locked "MOPA" system, (Master-Oscillator Power-Amplifier), having a phase coherent output beam.
The effect of combining identical phase-locked beamlets from a plurality of gain channels into a single beam is such that, in the limit of a very large number of individual channels, the resultant combined output beam exhibits an exceptional uniformity of illumination and intensity profile, far superior to that of any individual beamlet. As such, the performance of a radial geometry laser with MOPA approaches that of an ideal source, with high stability and near diffraction limited optical quality, at high average power.
The gain channels may be mounted in a toric resonator, which may include an output compacting axicon for uniform intensity laser beam extraction along the centerline and means for injecting an externally generated reference oscillator signal into the toric resonator system, for providing simultaneous injection-phase-locking of the plurality of gain channels, thereby providing near diffraction limited optical performance at high output power level.
A separate solid state cylindrical gain section (rod) may be mounted along the common central axis and pumped with laser excitation energy from the plurality of surrounding flashlamps or light emitting diode arrays. A portion of the coherent optical laser output radiation, produced by the centrally mounted gain section (rod) may be fed back into the plurality of radially mounted gain channels, thus providing a self-phase-locking feature for each individual radial gain channel and thereby yielding a self-injection-locked "MOPA" system, featuring a combined phase coherent output beam of high stability and near diffraction limited optical quality with high average power.
In a still further embodiment of the laser system, the radial gain channels are provided with Brewster Angle end sections, such that the optical cavity mode propagation executes a Zig-Zag path through each of the individual gain channels.